Pass formatter parameters to WKT writer

[kylebarron]

This is just a prototype of #199.

Based on the example here, this optionally passes down a precision parameter to the format!() call.

One thing I noticed when I hard-coded format!("{x:.3} {y:.3}") is that setting that forces even integers to have 3 decimal points. E.g.

---- geojson::geojson_reader::test::conversions stdout ----
thread 'geojson::geojson_reader::test::conversions' panicked at geozero/src/geojson/geojson_reader.rs:419:9:
assertion `left == right` failed
  left: "POINT(10.000 20.000)"
 right: "POINT(10 20)"

Potentially this means what I actually want to do to solve my goal in #199 is to truncate the precision of my data, and that will automatically get printed as I want...

[michaelkirk]

truncate the precision of my data, and that will automatically get printed as I want...

I think that approach will work (1.0 - 0.07) == 0.9299999999999999... percent of the time. 😉

[michaelkirk]

Dumb jokes aside, you mean something like this, right?

let y = (x * 10.pow(n)).round() / 10.pow(n);
format!("{}", y);

I think that should work 100 percent of the time, though opens us up to loss of precision at high values.

I don't know why, but I'm sort of surprised that a formatting option like this doesn't already exist.

[kylebarron]

Dumb jokes aside, you mean something like this, right?

let y = (x * 10.pow(n)).round() / 10.pow(n);
format!("{}", y);

Yeah. That's how I ended up implementing it in geoarrow: https://github.com/geoarrow/geoarrow-rs/blob/667389b2a1cdae704b815b6a447505027d876ff9/src/io/display/scalar.rs#L18-L21

opens us up to loss of precision at high values.

Hmm yeah that's true. I'm not sure how to truncate floats with high precision.

I don't know why, but I'm sort of surprised that a formatting option like this doesn't already exist.

You mean in core rust? Maybe it does; I haven't searched for it.

[frewsxcv]

fn main() {
    let num = 3.14159;
    let decimals = 3;
    let formatted = format!("{:.1$}", num, decimals);
    println!("{}", formatted);  // Output: 3.142
}

[kylebarron]

@frewsxcv that's what I'm doing here:
https://github.com/georust/geozero/pull/200/files#diff-553c2186acf21c8c360979841037ea9343c11e4e2937438cb9eca2c20d0d08cdR107-R109

But we'd still need a way for the user to pass down the precision desired